Neutralizing Antibodies Against St. Louis and Western Equine Encephalitic Viruses in Horses and Fowl

Abstract

It had previously been noticed that certain serums from healthy domestic fowl in California 1 showed neutralizing antibodies against the Br strain of western equine encephalomyelitic virus. Serums from areas where this disease is endemic appeared to yield more positive results than those from supposedly non-endemic regions. A correlation had also been observed with human serums between the presence of antibodies for both the western equine virus and that of the St. Louis encephalitic type. About 26% of 69 serums of encephalitic patients in the Central Valley areas of California were positive to both viruses. 2

To determine if a similar relationship existed with animals, neutralization tests were performed on serums of domestic fowl and horses collected from endemic and presumably non-endemic areas of California.† The same technic and the same strains of virus (Br and St. Louis) were used as previously described, 1 the serums being considered positive or weakly positive according to the number of virus dilutions neutralized.

It was interesting to find that antibodies against the St. Louis virus were present in the blood of many horses and domestic fowl and that, with but a few exceptions, they were in association with the neutralizing substance against the western equine virus. Of 43 unvaccinated normal horses and 2 recovered animals from the endemic regions of Kern, Fresno and Yolo Counties, all (95.3%) except 2 showed neutralizing antibodies against the western equine virus. With the exception of 10, the serums were positive to the St. Louis virus as well, but none was positive to the latter alone. Serums were tested from 19 horses vaccinated against the equine strain in a Northern California area. All were positive to the homologous virus but were negative to that of the St. Louis. Of 3 unvaccinated animals 2 were negative to both strains and one weakly positive to the equine.